class Solution 
{
    int dx[4] = {0, 0, 1, -1};
    int dy[4] = {1, -1, 0, 0};

public:
    int maxDistance(vector<vector<int>>& grid) 
    {
        int m = grid.size(), n = grid[0].size();
        vector<vector<int>> dist(m, vector<int>(n, -1));
        queue<pair<int, int>> q;

        // 1. 把所有的源点加入到队列里面
        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++)
                if(grid[i][j])
                {
                    dist[i][j] = 0;
                    q.push({i, j});
                }

        int ret = -1;
        while(q.size())
        {
            auto [a, b] = q.front();
            q.pop();
            for(int i = 0; i < 4; i++)
            {
                int x = a + dx[i], y = b + dy[i];
                if(x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)  // 没有被搜索过的
                {
                    dist[x][y] = dist[a][b] + 1;
                    q.push({x, y});
                    ret = max(ret, dist[x][y]); // 更新
                }
            }
        }

        return ret;
    }
};